A power converter causes a switching element such as a MOSFET or an IGBT to perform a switching operation and performs power conversion. When the power conversion is performed, a switching loss occurs according to the switching operation of the switching element. When the switching loss occurs, the temperature of the switching element (hereinafter referred to as “element temperature”) rises. Therefore, a cooling apparatus for suppressing the rise in the element temperature is indispensable.
In general, the cooling apparatus includes fins for cooling heat generation of an element. However, to further improve a cooling ability, there are many cooling apparatuses employing a boiling cooling system that makes use of a boiling phenomenon of a coolant incorporated therein.
In the cooling apparatus by the boiling cooling system (hereinafter referred to as “boiling cooling apparatus”), the coolant is filled inside the fins. The coolant boiled and vaporized by the heat generation is cooled by cooling air via a condenser, whereby the element is cooled.
The boiling cooling apparatus is usually fully filled with the coolant (e.g., water or flourinert). Pressure (steam pressure) obtained when the coolant vaporizes is about 0.3 atm at the normal temperature.
On the other hand, in the boiling cooling apparatus, it is known that fine holes are opened in a portion of the condenser under a normal environment of use. When the fine holes are opened, whereas the inside of the condenser is 0.3 atm, the outside of the condenser is 1 atm. Therefore, importance of managing mixing of the air in the atmosphere into the condenser is pointed out (e.g., Patent Literature 1).
Note that, in the boiling cooling apparatus described in Patent Literature 1 (in the literature, disclosed as “boiling cooling thyristor apparatus”), when the air, which is a noncondensable gas in the atmosphere, mixes in the condenser, a temperature difference occurs between upper and lower parts of the condenser. Therefore, the temperature difference is detected using a plurality of temperature sensors provided on the peripheral wall on the surface of the condenser. Cooling performance of the boiling cooling apparatus is grasped based on information concerning the detected temperature difference.